Inverse-square flash exposure

Distance doubles. Light becomes one quarter.

subject light = flash power / distance²

Subject light 1.00x relative to 4 ft at 1/16 power

Exposure shift 0.0 stops same as reference

Meter aperture f/5.6 same ISO and shutter speed

Power to match reference 1/16 at the selected distance

Distance ratio

Every doubling costs two stops.

Distance x2 1/4 light add x4 power, or +2 stops

Distance x4 1/16 light add x16 power, or +4 stops

Distance x0.5 x4 light reduce power by two stops